Vibrating structure gyroscopes function as a result of developed Coriolis forces which occur when a particle undergoes linear motion in a rotating frame of reference. Practical implementations of this technology are described in U.S. Pat. No. 5,226,321 and U.S. Pat. No. 4,655,081. U.S. Pat. No. 5,226,321 discloses the use of a planar hoop or ring like structure for the vibrating element or resonator of such a vibrating structure gyroscope, which resonator structure may be conventionally machined or micromachined using silicon processing technology.
These planar structures are typically excited into a cos 2.theta. resonance mode. For a perfectly symmetric resonator, the cos 2.theta. mode actually exists as a degenerate pair of vibration modes at a mutual angle of 45.degree.. These are shown schematically in FIGS. 1A and 1B in which vibration of the structure is shown about primary axes P and secondary axes S. Thus FIGS. 1A and 1B show the resonator shape at the two points of extreme deformation during a single vibration cycle.
One of these modes (FIG. 1A) is excited as the carrier mode. When the structure is rotated around the axis normal to the plane of the ring, Coriolis forces couple energy into the second response mode (FIG. 1B) The amplitude of motion of the response mode gives a direct measure of the applied rotation rate.
U.S. Pat. No. 5,226,321 further discloses that the resonator may be driven into excitation by various means, preferably electromagnetic, but including optical, thermal expansion, piezo-electric or electrostatic effects, with vibration of the resonator being sensed preferably by electrostatic (capacitative) means.
Such a known structure has a number of disadvantages. Firstly the capacitative vibration sensing means or pick-off are sensitive to vibration in the plane perpendicular to the plane of vibration of the resonator which can make the gyro sensitive to vibration inputs. Secondly the use of capacitative pick-offs necessitates the use of very small gaps between the pick-offs and vibrating resonator in order to achieve sensitivity. Small gaps cause a yield problem during manufacture from silicon wafers because of the problem of stiction which prevents the resonator part from releasing from the remainder of the silicon structure during the manufacturing etch process. A further disadvantage is that the design is asymmetric and this requires subsequent orientation of the sensing means or pick-offs during final assembly. Asymmetry in the resonator structure further contributes to the splitting of the Cos 2.theta. mode frequencies which degrades sensor performance.